This invention relates to spacers for coaxially arranged tube systems where a temperature difference exists between individual tubes. Specifically the tubes are spaced from each other by helical spacers which include ribbons or strips of insulating material and having low thermal conductivity, these strips being so arranged that they touch the respective tubes only at individual areas.
It is already known, for example, to utilize cable cooled to very low temperatures for transmitting electric energy. In this case the conductor consists of a tube the interior of which is cooled by liquid helium. In order to protect the tube from externally supplied heat, additional tubes are provided which concentrically surround the central tube and which are spaced from each other. A vacuum may be maintained in a space between the innermost and the adjacent tube. The separation between the second and third tube is conventionally filled with liquid nitrogen in order to control the temperature drop towards the interior. The space between the third and the last tube may again be evacuated and this last or outermost tube has ambient temperature. Similarly arranged tube systems which may not be cooled with liquid nitrogen may also be utilized for transporting liquified gases or other media at very low temperatures. Essential for all these constructions is the fact that the heat loss either by conduction of heat or by radiation is reduced to a minimum without which economical operation of such systems would not be possible.
To this end innumerable proposals have been made for spacers providing a minimum of heat loss. For example, the spacer may be arranged in the form of helical coils surrounding the innermost tube or disc shaped supporting elements to support the tubes in concentric relationship.
U.S. letters Pat. No. 3,592,238 and No. 3,670,772 disclose spacer constructions for coaxial tube systems in which the helical spacer is constructed from individual ribbons which are loosely superimposed upon each other. The ribbons or strips may additionally be twisted in the form in which they are used in order to further reduce the heat transfer from one tube to another.